In the operation of a screening machine, irregular particles--especially hard, rough materials such as wood chips--which are a little too large to pass all the way through the screen openings sometimes become stuck in screen openings. These effectively close the openings, thereby reducing the area, and hence the capacity, of the screen. Commercial screening machines often incorporate what are called "screen cleaners" which strike the screen while the machine is operating so as to dislodge particles stuck in the screen openings. In one common type of screen cleaner, elastomeric balls are confined in small pockets or "cages" directly beneath the screen and are caused to bounce up against the bottom of the screen to dislodge the particles. In some instances the cage walls are beveled or angulated upwardly; the screening motion of the machine causes the balls to move around in the cages and bounce upwardly off the angled walls, against the screen. The cages have mesh floors with large openings so that particles which fall through the screen above will fall through the cages to an outlet below or onto a finer screen for further screening. Examples of such "ball cleaners" are shown in Holcomb U.S. Pat. No. 218,530; Dawson U.S. Pat. No. 567,963; Dimm U.S. Pat. No. 1,562,311; Simpson U.S. Pat. No. 2,226,416; and Frevert U.S. Pat. No. 4,498,981.
In many situations commercial machines are required to run for extended periods without shut down. Eventually the vigorous bouncing action of the balls on the cage walls and floor wears or erodes the balls and the cage itself, as well as the screen. The wear is most rapid on the screen itself, but continued abrasion of the balls on the floor and the walls of the cage (which are often made of wood) gradually weakens the cage. The cage walls underlie and support the screen, which rests on the tops of the walls. If the walls are eroded too much they can no longer support the weight of the particulate material on the screen, and they may collapse. To prevent that, the entire ball cleaner must be replaced at intervals, which is a significant delay and expense. In some cases metal cages have been used rather than wood, but they are more expensive and it is more difficult to mount a wire mesh floor to them.
Pennington U.S. Pat. No. 3,565,251 attempts to solve the problem of cage wear by providing a plastic cage insert which is surrounded by a supporting peripheral frame. When the plastic is worn, the entire insert can be removed and replaced. The weight of the screen and particle load on the screen rests on and is carried by the plastic insert. The insert must be heavy to withstand the loading, which increases its cost, and as it wears its ability to carry the load decreases. Moreover, engagement of the top edge of the cage walls with the screen increases the rate of wear of the insert.